Linking microbially mediated soil organic matter turnover and nitrogen mineralisation to phosphorus and pH management in grassland soils
Kelly, Sorcha
Kelly, Sorcha
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Publication Date
2022-01-10
Type
Thesis
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Abstract
Nitrogen (N) is an essential nutrient for plant growth that is often limiting in agricultural production systems. While soil organic matter (SOM) represents a large terrestrial store of N, N stored in SOM is not directly available for plant uptake. The conversion of organic N in SOM to inorganic (plant-available) N is dependent on microbial activity, which in turn is influenced by plant-derived organic inputs to soil (rhizodeposition). Predicting when and in what form N becomes available from SOM can enhance farm nutrient management practices by linking nutrient cycling with plant growth. This has benefits agronomically and economically, by reducing reliance on expensive N fertilisers; and environmentally, by reducing nutrient losses to waterways and as greenhouse gas emissions, as N use becomes more soil, field and crop specific. It is currently poorly understood how phosphorus addition, in both inorganic and organic forms, and soil pH management, through lime addition, impacts the processes involved in SOM N turnover. The main aim of this thesis was to address this knowledge deficit by investigating how these soil management practices impact the plant-soil-microbial processes (i.e., SOM decomposition and priming) that are involved in N mineralisation, and the associated changes in the microbial community. This was achieved through several hypotheses-driven experiments using analytical methods, novel in their combination of use, including a seven-day anaerobic assay (for the measurement of potential N mineralisation), stable 13C isotope labelling through both 13CO2 and 13C glucose solution additions (for the partitioning and quantification carbon (C) fluxes in the soil system), and high throughput Illumina sequencing of 16S and ITS genes. The impact of P management on N mineralisation potential was inconsistent, and long-term P addition had the capacity to either increase or decrease N mineralisation. However, when P was applied in organic form N mineralisation consistently increased. Higher P input caused a greater magnitude of priming (both positive and negative) compared to treatments which received less P. High P addition in the organic form reduced SOM decomposition rates, compared to when P was applied in an inorganic form, and resulted in a negative priming response. Therefore, there was a decrease in the release of C from SOM in this treatment, which will have implications for both increasing soil health and the sustainability of organic nutrient addition. P addition, both in the organic and inorganic form, had an impact on the bacterial and fungal microbial community structure. Overall, N mineralisation potential increased with pH and this was associated with a shift in microbial community structure. This was particularly evident with the prokaryotic (16S) community, where there was an increase in diversity with increasing pH. For both the 16S and fungal communities, the largest variations in abundance were seen between treatments where the differences in pH were greatest. Unexpectedly, the lime-derived CO2 emissions could be detected more than 12 months after field application of lime. This has potentially important implications when considering the C budget of agricultural soils in grassland systems, where lime is typically a common agronomic management strategy and requires further research. This thesis highlights the effects of soil management on C and N cycling in grassland soils and the findings will contribute towards sustainable management practices in grassland ecosystems to promote soil health and reduce nutrient losses to the environment.
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NUI Galway